Oracle does not implement the POSIX ERE standard exactly, however. It deviates in three areas. First, Oracle supports the backreferences\1 through \9 in the regular expression. The POSIX ERE standard does not support these, even though POSIX BRE does. In a fully compliant engine, \1 through \9 would be illegal. The POSIX standard states it is illegal to escape a character that is not a metacharacter with a backslash. Oracle allows this, and simply ignores the backslash. E.g. \q is identical to q in Oracle. The result is that all POSIX ERE regular expressions can be used with Oracle, but some regular expressions that work in Oracle may cause an error in a fully POSIX-compliant engine. Obviously, if you only work with Oracle, these differences are irrelevant.

The third difference is more subtle. It won't cause any errors, but may result in different matches. As I explained in the topic about the POSIX standard, it requires the regex engine to return the longest match in case of alternation. Oracle's engine does not do this. It is a traditional NFA engine, like all non-POSIX regex flavors discussed on this website.

If you've worked with regular expressions in other programming languages, be aware that POSIX does not support non-printable character escapes like \t for a tab or \n for a newline. You can use these with a POSIX engine in a programming language like C++, because the C++ compiler will interpret the \t and \n in string constants. In SQL statements, you'll need to type an actual tab or line break in the string with your regular expression to make it match a tab or line break. Oracle's regex engine will interpret the string '\t' as the regex t when passed as the regexp parameter.

Oracle’s REGEXP Functions

Oracle Database 10g offers four regular expression functions. You can use these equally in your SQL and PL/SQL statements.

REGEXP_LIKE(source, regexp, modes) is probably the one you'll use most. You can use it in the WHERE and HAVING clauses of a SELECT statement. In a PL/SQL script, it returns a Boolean value. You can also use it in a CHECK constraint. The source parameter is the string or column the regex should be matched against. The regexp parameter is a string with your regular expression. The modes parameter is optional. It sets the matching modes.

REGEXP_SUBSTR(source, regexp, position, occurrence, modes) returns a string with the part of source matched by the regular expression. If the match attempt fails, NULL is returned. You can use REGEXP_SUBSTR with a single string or with a column. You can use it in SELECT clauses to retrieve only a certain part of a column. The position parameter specifies the character position in the source string at which the match attempt should start. The first character has position 1. The occurrence parameter specifies which match to get. Set it to 1 to get the first match. If you specify a higher number, Oracle will continue to attempt to match the regex starting at the end of the previous match, until it found as many matches as you specified. The last match is then returned. If there are fewer matches, NULL is returned. Do not confuse this parameter with backreferences. Oracle does not provide a function to return the part of the string matched by a capturing group. The last three parameters are optional.

REGEXP_REPLACE(source, regexp, replacement, position, occurrence, modes) returns the source string with one or all regex matches replaced. If no matches can be found, the original string is replaced. If you specify a positive number for occurrence (see the above paragraph) only that match is replaced. If you specify zero or omit the parameter, all matches are replaced. The last three parameters are optional. The replacement parameter is a string that each regex match will be replaced with. You can use the backreferences\1 through \9 in the replacement text to re-insert text matched by a capturing group. You can reference the same group more than once. There's no replacement text token to re-insert the whole regex match. To do that, put parentheses around the whole regexp, and use \1 in the replacement. If you want to insert \1 literally, use the string '\\1'. Backslashes only need to be escaped if they're followed by a digit or another backslash. To insert \\ literally, use the string '\\\\'. While SQL does not require backslashes to be escaped in strings, the REGEXP_REPLACE function does.

REGEXP_INSTR(source, regexp, position, occurrence, return_option, modes) returns the beginning or ending position of a regex match in the source string. This function takes the same parameters as REGEXP_SUBSTR, plus one more. Set return_option to zero or omit the parameter to get the position of the first character in match. Set it to one to get the position of the first character after the match. The first character in the string has position 1. REGEXP_INSTR returns zero if the match cannot be found. The last 4 parameters are optional.

SELECT REGEXP_INSTR(mycolumn, 'regexp', 1, 1, 0, 'i') FROM mytable;

REGEXP_COUNT(source, regexp, position, modes) returns the number of times the regex can be matched in the source string. It returns zero if the regex finds no matches at all. This function is only available in Oracle 11g and later.

SELECT REGEXP_COUNT(mycolumn, 'regexp', 1, 'i') FROM mytable;

Oracle’s Matching Modes

The modes parameter that each of the four regexp functions accepts should be a string of up to three characters, out of four possible characters. E.g. 'i' turns on case insensitive matching, while 'inm' turns on those three options. 'i' and 'c' are mutually exclusive. If you omit this parameter or pass an empty string, the default matching modes are used.

'i': Turn on case insensitive matching. The default depends on the NLS_SORT setting.

'c': Turn on case sensitive matching. The default depends on the NLS_SORT setting.

'n': Make the dot match any character, including newlines. By default, the dot matches any character except newlines.

'm': Make the caret and dollar match at the start and end of each line (i.e. after and before line breaks embedded in the source string). By default, these only match at the very start and the very end of the string.

'x': Turn on free-spacing mode which ignores any unescaped whitespace outside character classes (10gR2 and later).

Further Reading

O'Reilly's book Oracle Regular Expressions Pocket Reference is a very handy 64-page volume that tells you everything you need to know about regular expressions in Oracle Database 10g. Despite the book's cover, it actually contains both a tutorial and a reference. Since Oracle's regular expression support is fairly limited, this small book is all you need to successfully use regular expressions with Oracle.

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